1. Field of the Invention
The present invention relates to an apparatus for mirror-polishing a thin plate. More particularly, the invention relates to an apparatus for mirror-polishing a thin plate used for mirror-polishing a semiconductor wafer (hereinafter referred to as wafer) to a high flatness.
2. Related Art
In a fabrication process of a semiconductor integrated circuit, for example, a design rule applied to a device has been narrowed and a depth of focus in a stepper in a photolithography step has become shallower in the trend of ever increasing integration of the device. Accordingly, requirement for a higher flatness of a wafer has also become more demanding in a mirror-polishing process for the wafer.
Conventionally, a mechano-chemical polishing method, which is a combination of mechanical polishing and chemical polishing, has been used in an apparatus for mirror-polishing a wafer.
In this mechano-chemical polishing method, a wax method, for example, has been used, comprising: fixing with wax a wafer on a lower surface of a support mounted on a freely rotatable, press member; and subjecting the wafer to abrasion with an unwoven polishing pad, as a plashing means, fixed on a major surface of a turn table, while being pressed. In the wax method, however, there are technical limitations to improvement on a flatness of the wafer, since it is difficult to apply the wax on the lower surface of a support uniformly and foreign matter invades into a gap between the wafer and the lower surface of a support, so that generation of dimples can not be avoided.
A waxless method has been also used, in which a wafer is held on a lower surface of the support with a backing pad in a state of sponge lying therebetween and the wafer is polished in a similar way to that of the polishing process in the above mentioned wax method. In this waxless method, however, there is also a problem that pores on the surface of the backing pad in the state of sponge are loaded with solids in polishing slurry or the like and an elasticity of the pad is locally changed, so that a flatness of the wafer is degraded. Accordingly, in the waxless method, there is a requirement that frequent exchanges of backing pads are indispensable in order to prevent degradation in flatness of the wafer.
In consideration of such circumstances, an apparatus for mirror-polishing, which adopts an elastic film holding method, as disclosed in Laid-open Japanese Patent Application No. 5-69310, has been recently proposed. This apparatus employs, as shown in FIG. 4, an elastic film 91 with flexibility on a supporting surface thereof for the wafer. As an material of the elastic film 91, a sheet made of synthetic rubber such as silicone rubber or the like, or a reinforced rubber or reinforced vinyl resin with any of fibers and cloth or the like is used. In such an apparatus for mirror-polishing, the elastic film 91 is fastened on a cylinder-like holder body 92 with application of uniform tension, a surface (back surface) of the film opposed to the surface thereof, on which the wafer is held, is exposed to a pressure adjusting fluid for pressing the wafer, said fluid is supplied from a fluid feed/exhaust apparatus, not shown. A ring 94 is mounted on the outer periphery of the holder 92, as shown in FIG. 5, in such a manner that the ring 94 is free to movable vertically, the thin elastic film 91 is adhered or mechanically fixed in a squeezing manner on the film 94 and the ring 94 is adjustably moved upward or downward with a screw 95, so that the thin elastic film may be tightened in a uniform manner. A mark 96 indicates a polishing pad fixedly held on a major surface of a turn table.
The above apparatus for mirror-polishing still has a problem. In order to attain a uniform polishing stock removal on a wafer, conditions of tightness of the elastic film 91 are important. In the apparatus employing the elastic film holding method, the screw 95 works for vertical shifts of the ring 94 in order to adjust the tightness of the elastic film 91 but it is difficult to properly determine a position itself of the ring on the outer surface of the holder 92 by adjusting a degree of clamping-down of each screw 95 and besides it is more difficult to uniformalize clamping-down degrees of screws 95, said screws 95 being located on the outer periphery of the holder 92. For the above reason, in the apparatus, uniform tightness across the elastic film 91 working as a holding surface for a wafer W has not been achieved. In the apparatus, moreover, since the outer periphery of the holder 92 and the inner periphery of the ring 94 are respectively uniform in diameter, a gap is easy to be created between the outer periphery of the holder 92 and the inner periphery of the ring 94, which leads to another problem that such a structure has poor gastightness or fluidtightness.
Such problems are not restricted to the case of a semiconductor wafer but generally occur in mirror-polishing of a thin plate.